Stretch wrap films of the prior art are used in connection with a wide variety of overwrap packaging applications. In many applications, these films are required to be air and moisture vapor permeable materials. Techniques have been developed to introduce through-holes (i.e., perforations) into the film for such a purpose. In order to make a thin film with perforations, the prior art generally included a stretching apparatus to thin-out the film and a separate “punching” apparatus for introducing perforations into the stretched film. One exemplary method of producing such films is disclosed in U.S. Pat. No. 6,296,469 issued to M. Suzuki et al. on Oct. 2, 2001. In the Suzuki et al. apparatus, a pair of punching rollers is used which sandwich the film between the rollers.
U.S. Pat. No. 4,765,120, issued to T. E. Phillips on Aug. 23, 1988 discloses a device for perforating a film while wrapping a load. In particular, the Phillips arrangement includes electrodes positioned proximate to the film and an arrangement for generating an arc across the electrodes to form the perforations in the film. Such an arrangement is not considered to be well-suited for situations where the presence of constant arcing may damage the product being wrapped, the wrapping machine and/or other products/machines in the general area of the arcing equipment.
Another type of perforating apparatus is disclosed in U.S. Pat. No. 5,802,945, issued to F. Brinkimeier et al. on Sep. 8, 1998, which describes the use of a series of “needles” to introduce perforations into a roll of film. In order to be able to create a variety of perforation patterns and/or holes of varying diameter in the film, a needle roller arrangement is used that includes several separate needle rollers arranged in a freely rotatable manner in a rotary frame. One of the needle rollers interacts with a brush roller for perforating a film which passes between the brush roller and the needle roller.
U.S. Pat. No. 5,935,681 issued to H. K, Paulett on Aug. 10, 1999 discloses another arrangement for forming “air permeable” stretch film, where in this case the film comprises separate first and second layers of polymeric film (linear low density polyethylene) which is capable of stretching at least 150% beyond its original length. The layers will naturally cling together to form a laminate. Perforations are formed in the laminate by applying a “hot pin” to the laminate, whereby the polymeric film is melted. As the melt hardens, the perforations become defined by the welds, or reinforcements, which add strength to the film.
Laser-based systems have also been developed for creating perforations of a known, controlled sized along a roll of wrapping film (usually a plastic material, although other film materials may be used). See, for example, U.S. Pat. No. 6,730,874 issued to E. Varriano-Marston on May 4, 2004. In operation, a roll of plastic film is fed into a laser perforation apparatus, where the film is unrolled and perforated and thereafter taken up on an output reel. The laser-perforated roll is then ready for use at the food packaging machine.
One problem with many of these film perforation arrangements is that the perforating apparatus is generally not co-located with the items being wrapped; delays may occur in obtaining the perforated film at the plant where the wrapping is taking place. Further, the need to completely unroll and then re-roll the plastic film during the prior art perforation processes tends to create stress across and along the film, creating folds, offsets in the wrapped layers, and the like, which may lead to tears or other problems during the actual wrapping process.
Thus, a need remains in the art for an improved arrangement for creating perforations in the film used to wrap food products.